Integrated circuits require a power supply to operate. When the circuit is not operating, the power supply is typically shut off. However, there are certain circuit elements for whom some level of power (such as battery power) is required at all times, even if the main power is shut down. For example, volatile random access memory (RAM) arrays, also known as "static RAMs" or SRAMs require some power to maintain the information stored therein, if such is necessary.
Power supply switches which select between a main power supply and a battery backup supply are well known in the art. These switches continually compare the voltage levels of the two supplies and switch to the battery supply when the voltage of the main power supply falls below the level of the battery supply. The switch switches to the main supply once its voltage level is above that of the battery supply.
For example, U.S. Pat. No. 4,730,121 to Lee et al. describes one battery backup switching circuit which, in addition to the switching circuit, includes circuitry for receiving a reset or isolation signal. If the isolation signal is active, when the main power falls, the circuit will be isolated from both power sources.
U.S. Pat. No. 5,315,549 to Scherpenberg et al. describes a memory controller which supplies backup battery power when the main power supply fails.
There are some logic elements, particularly those of the switching circuit and often other elements as well, which must continually receive some level of power. The switching circuit elements require the power in order to independently determine when the main power has returned. The other logic elements may require power in order to provide logic signals during battery power operation.
Unfortunately, the continually operative logic elements continually pull power from the battery, reducing the life of the battery. Accordingly, integrated circuit designers work to reduce the number of parts which utilize battery power.
It is possible that the part with the VLSI circuit will operate on battery power for a significant length of time, particularly when the part is stored in inventory, prior to be used for the first time. The battery might then be significantly rundown even before the part is ever utilized.
U.S. Pat. No. 4,908,790 to Little et al. describes a backup battery switching circuit which selectively connects the backup battery only when it is desired to preserve the data in the RAM. The microcomputer to which the RAM is connected controls the switching circuit of Little et al. and commands whether or not the backup battery is to be utilized.